1. Field of the Invention
The present invention relates to improvement of a magnetostrictive torque sensor for detecting a steering force and in particular, to suppress a magnetic leak and improvement of a mechanical strength.
2. Description of the Related Art
A steering force detecting magnetostrictive torque sensor is already known for driving and controlling a power steering system by detecting a steering force functioning on a steering shaft so as to output a torque detection signal.
This type of magnetostrictive torque sensor includes: a magnetic-anisotropy at the surface of the sensor shaft that connecting a steering shaft; and an excitation coil and a detection coil arranged around the magnetic-anisotrophy, so that a torsion generated at the surface of the sensor shaft according to the steering torque by a driver is detected as a change of magnetic permeability of the magnetic anisotropy, thereby detecting a steering force functioning on the steering shaft.
Because a conventional steering force detecting magnetostrictive torque sensor is not provided with particular magnetic shield means, the magnetostrictive torque sensor attached to a vehicle is affected by a change of external magnetic environment. For this, at the stage before attaching the torque sensor, there is a problem that it is difficult to adjusting a zero-torque signal (an output when no external force is applied) of the sensor.
In order to solve this problem, there has been suggested a steering force detecting magnetostrictive torque sensor including a detection coil and an excitation coil having an outer circumference covered by a shield formed from a non-magnetic high-conductive material, thereby preventing a magnetic leak from the detection coil and the excitation coil as well as an external noise so as to obtain a magnetically stable environment around the sensor.
As shown in FIG. 5, in this steering force detecting magnetostrictive torque sensor, as shown in FIG. 5, has an outer circumference covered by a shield 102 formed from a non-magnetic high-conductive material, thereby suppressing a magnetic leak from the detection coil 101 and the outer circumference of the shield 102 is covered by a yoke 103 formed by a soft-magnetic material, thereby preventing an external noise.
However, since the shield 102 of a non-magnetic and high conductive material is arranged in close contact to the outer circumference of the detection coil 101, the energy loss due to heat dissipation is great, lowering the sensitivity of the magnetostrictive torque sensor.
Moreover, since the yoke 103 is mounted in an exposed stated, an external force may be applied to this portion, which changes a magnetic characteristic of the yoke 103. That is, a special attention should be paid in handling.
Furthermore, for mounting the detection coil 101 and shield 102, a coil bobbin 100 is formed with variable diameter values. The sensor shaft 105 is fixed as a unitary block to this coil bobbin via a bearing 104. Accordingly, when an excessive load is applied to this coil bobbin 100 by eccentricity of the steering shaft and the steering output shaft, the coil bobbin 100 itself may be deformed or scratched. Furthermore, relative positional changes are caused between the detection coil 101, the shield 102, the yoke 103, and the like, causing a fluctuation of the detection characteristic.
It is therefore an object of the present invention to provide a steering force detecting magnetostrictive torque sensor magnetically stable and having a sufficient mechanical strength and a reduced energy loss due to heat dissipation.
The present invention provides a steering force detecting magnetostrictive torque sensor comprising: a sensor shaft mounted between a steering shaft and a steering output shaft; a magnetic anisotropy at the surface of the sensor shaft; a coil bobbin wound by an excitation coil and a detection coil to surround the magnetic anisotropy; and a yoke surrounding the coil bobbin, the steering force detecting magnetostrictive torque sensor further comprising a casing including a casing main body formed from a non-magnetic high conductive material as a container having one side opening and an access panel to cover the opening, wherein the coil bobbin and the yoke constitute a magnetism detection unit, which is arranged inside the casing main body, and a through hole is arranged at two end portions in the axial direction of the magnetism detection unit in the casing main body for inserting the sensor shaft, so that the sensor shaft is rotatably attached to the through holes via a bearing.
The casing main body made from a non-magnetic high conductive material and having the magnetism detection unit function as a magnetic shield, so as to suppress magnetic leak from the detection coil and the excitation coil as well as prevent intrusion of an external noise. This eliminates an external magnetic environment change caused before and after mounting the magnetostrictive torque sensor to a vehicle.
Moreover, a sufficient space can be obtained around the excitation coil and the detection coil in the casing body covered by the access panel. Accordingly, it is possible to reduce the energy loss such as heat generation caused by interaction between the coils and the casing main body and the access panel made from a non-magnetic high conductive material, thereby preventing lowering sensitivity of the magnetostrictive torque sensor.
Moreover, the yoke arranged at the outermost portion of the magnetism detection unit is protected by the casing main body and the access panel. Accordingly, there is no possibility of application of an unnecessary external force to the yoke and it becomes easier to handle the entire apparatus.
Furthermore, the sensor shaft having the magnetic anisotropy is rotatably attached via bearings to through holes arranged on the two walls of the casing main body positioned at the both end portions of the magnetism detection unit in the axial direction. Accordingly, even when eccentricity is present between the steering shaft and the sensor shaft or between the steering output shaft and the sensor shaft, there is no danger of deforming or scratching the excitation coil or the coil bobbin having the detection coil and the yoke by an external force. Thus, it is possible to obtain a sufficient mechanical strength and to eliminate characteristic changes due to fluctuations of a positional relationship between the members.
The access panel covering the opening of the casing main body may be formed by a multi-layered circuit board on which an electric component is mounted for processing a signal from the detection coil.
This type of multi-layered circuit board includes a copper foil layer in the board itself and accordingly, can be used in place of the magnetic shield formed from a non-magnetic high conductive material. That is, there is no need to provide a dedicated access panel formed from a non-magnetic high conductive material and the circuit board serves also as the magnetic shield. This reduces the cost of the entire apparatus.
Moreover, it is also possible to insert a shield panel formed from a non-magnetic high conductive material between the access panel and the magnetism detection unit.
In this case, the cost is slightly increased as compared to a case using only the copper foil layer of the multi-layered circuit board itself as the magnetic shield. However, it becomes possible to assure a shield effect, thereby further improving magnetic stability of the magnetostrictive torque sensor.
Furthermore, the bearing supporting the sensor shaft may be an ordinary bearing or a bush made from a non-magnetic high conductive material.
By utilizing a bush made from a non-magnetic high conductive material instead of an ordinary bearing, it is possible to further reduce magnetic leak and noise, thereby improving the magnetic stability of the magnetostrictive torque sensor. As the non-magnetic high conductive material for the casing main body and the shield panel, it is preferable to employ an aluminum alloy or the like, considering the weight and machining feasibility. The bush serving as the bearing is preferably formed from a JIS PBC2 material, considering the abrasion resistance.
Moreover, a convex/concave engagement portion is formed on inner sides of two walls of the casing main body positioned at both end portions of the magnetism detection unit in the axial direction and at both end portions of the magnetism detection unit in the axial direction for positioning the magnetism detection unit with respect to the casing main body, and the both end portions of the magnetism detection unit in the axial direction are fixed by an adhesive to the two inner sides of the two walls.
Thus, with the configuration of accurately positioning the magnetism detection unit with respect to the casing main body, it is possible to properly maintain a positional relationship of the excitation coil and the detection coil against the sensor shaft mounted onto the casing main body via a bearing or a bush.